PURPOSE: To discriminate a cylinder group and detect a crank angle stably based on a few memory data when a rotation change is big like the start time of an internal combustion engine.

CONSTITUTION: In the rotary body 1 installed on the crank shaft 6 of an internal combustion engine, projections 2, 3, 4, 2', 3', 4' at a prescribed angle position as for a crank angle discrimination and projections 5, 5' at a prescribed angle position as for a cylinder group discrimination are formed respectively on its outer periphery. It is discriminated whether or not a discrimination function T12-K(T0×T2) is more than 0 by using continuous three signal intervals T2, T1, T0 and a prescribed value K of an angle signal outputted from an electro- magnetic pick up sensor 10 based on the rotation of the rotary body 1. In this case, a cylinder group can be discriminated and a crank angle can be detected stably even when the rotation change is big, as the discrimination function is more than 0 at only the point of an angle signal detection time at the projection 5 due to that the signal interval between the projection 2' and the projection 4 is long.